1. Field of the Invention
This invention relates to a microstructure array, such as a microlens array that is usable in the field of optoelectronics and the like, a mold or a master of a mold (in the specification the term “mold” is chiefly used in the broad sense including both a mold and a master of a mold) for forming a microstructure array, a fabrication method of the microstructure array, and so forth.
2. Description of the Related Background Art
A microlens array typically has a structure of arrayed minute lenses each having a diameter from about 2 to 3 microns to about 200 or 300 microns and an approximately spherical profile. The microlens array is usable in a variety of applications, such as liquid-crystal display devices, light receivers and interfiber connections in optical communication systems.
Meanwhile, earnest developments have been made to develop a surface emitting laser and the like that can be readily arranged in an array form at narrow pitches between the devices. Accordingly, there exists a significant need for a microlens array with narrow lens intervals and a large numerical aperture (NA).
Likewise, a light receiving device, such as a charge coupled device (CCD), has been increasingly downsized as semiconductor processing techniques develop and advance. Therefore, also in this field, the need for a microlens array with narrow lens intervals and a large NA is increasing. In the field of such a microlens, a desirable structure is a microlens with a large light-condensing efficiency that can highly efficiently utilize light incident on its lens surface.
Further, similar needs exist in prospective fields of optical information processing, such as optical parallel processing-operations and optical interconnections.
Furthermore, display devices of active or self-radiating types, such as electroluminescent (EL) panels, have been enthusiastically studied and developed, and a highly-defined and highly-luminous display has been thus proposed. In such a display, there is a heightened need for a microlens array that can be produced at a relatively low cost and with a large area, as well as with a small lens size and a large NA.
There are presently a number of conventional methods for fabricating microlenses. In a conventional microlens-array fabrication method using an ion exchange method (see M. Oikawa, et al., Jpn. J. Appl. Phys. 20(1) L51-54, 1981), the refractive index is increased at plural locations in a substrate of multi-component glass by using an ion exchange method. A plurality of lenses are thus formed at high-refractive index locations. In this method, however, the lens diameter cannot be large, compared with intervals between lenses. Hence, it is difficult to design a lens with a large NA.
Further, the fabrication of a large-area microlens array is not easy, since a large scale manufacturing apparatus, such as an ion diffusion apparatus, is required to produce such a microlens array. Moreover, an ion exchange process is needed for each glass, in contrast with a molding method using a mold. Therefore, variations of lens quality, such as a focal length, are likely to increase between lots unless the management of fabrication conditions in the manufacturing apparatus is carefully conducted. In addition to the above, the cost of this method is relatively high, as compared with the method using a mold.
Further, in the ion exchange method, alkaline ions for ion-exchange are indispensable in a glass substrate, and therefore, the material of the substrate is limited to alkaline glass. The alkaline glass is, however, unfit for a semiconductor-based device which needs to be free of alkaline ions. Furthermore, since the thermal expansion coefficient of the glass substrate greatly differs from that of a substrate of a light radiating or receiving device, misalignment between the microlens array and the devices is likely to occur due to a mismatch between their thermal expansion coefficients as the integration density of the devices increases.
Moreover, a compressive strain inherently remains on the glass surface which is processed by the ion exchange method. Accordingly, the glass tends to warp, and hence, the difficulty in joining or bonding between the glass and the light radiating or receiving device increases as the size of the microlens array increases.
In another conventional method, an original plate of a microlens is fabricated, lens material is deposited on the original plate and the deposited lens material is then separated. The original plate or mold is fabricated by an electron-beam lithography method (see Japanese Patent Application Laid-Open No. 1(1989)-261601), or a wet etching method (see Japanese Patent Application Laid-Open No. 5(1993)-303009). In these methods, the microlens can be reproduced by molding, variations between lots are unlikely to occur, and the microlens can be fabricated at a low cost. Further, the problems of alignment error and warping due to the difference in the thermal expansion coefficient can be solved, in contrast with the ion exchange method. In the electron-beam lithography method, however, an electron-beam lithographic apparatus is expensive and a large investment in equipment is needed. Further, it is difficult to fabricate a mold having a large area more than 100 cm2 (10 cm-square) because the electron beam impact area is limited.
In another conventional method, a mask layer with serially or two-dimensionally arranged openings is formed on a mother substrate, and etching is performed through the openings (see Japanese Patent Application Laid-Open No. 8(1996)-136704). In this method, however, since the etching is conducted through the resist opening, the bottom of a dug portion inevitably becomes flat and it is hence difficult to condense light into an area less than the area of the opening. Further, in a wet etching method, since an isotropic etching using a chemical action is principally employed, formation of the mother substrate into a desired profile cannot be achieved if the composition and crystalline structure of the mother substrate vary even slightly. In addition, etching will continue unless the plate is washed immediately after a desired shape is obtained. When a minute microlens is to be formed, a deviation of the shape from a desired one is possible due to etching lasting during a period from the time a desired profile is reached to the time the microlens is reached.
In yet another conventional microlens-array fabrication method using a resist reflow (or melting) method (see D. Daly, et al., Proc. Microlens Arrays Teddington., p 23-34, 1991), resin formed on a substrate is cylindrically patterned using a photolithography process and a microlens array is fabricated by heating and reflowing the resin. Lenses having various shapes can be fabricated at a low cost by this resist reflow method. Further, this method has no problems of thermal expansion coefficient, warping and so forth, in contrast with the ion exchange method.
In the resist reflow method, however, the profile of the microlens is strongly dependent on the thickness of the resin, the wetting condition between the substrate and the resin, and the heating temperature. Therefore, variations between lots are likely to occur while fabrication reproducibility per a single substrate surface is high. In addition, if adjacent resin comes in contact due to the reflow, the resist cannot maintain its desired profile due to surface tension. Therefore, it is hard to fabricate a microlens array in which adjacent microlenses are brought into contact and unusable regions between microlenses are reduced to increase the light-condensing efficiency.